Method of cyclically developing electrostatic latent images

ABSTRACT

AN ELECTROSTATOGRAPHIC IMAGING SYSTEM WHEREIN AN ELECTROSTATIC LATENT IMAGE IS DEVELOPED BY PLACING THE IMAGEING SURFACE IN DEVELOPING CONFIGURATION WITH A PATTERNED APPLICATOR SURFACE HAVING A SUBSTANTIALLY UNIFORM DISTRIBUTION OF RAISED PORTIONS OR &#34;LANDS&#34; AND DEPRESSED PORTIONS OR &#34;VALLEYS&#34; AND CONTAINING A RELATIVELY NON-CONDUCTIVE LIQUID DEVELOPER IN THE DEPRESSED PORTIONS THEREOF WHILE THE RAISED PORTIONS ARE SUBSTANTIALLY FREE OF DEVELOPER.

United States Patent US. Cl. 117-37 LE 22 Claims ABSTRACT OF THEDISCLOSURE An electrostatographic imaging system wherein anelectrostatic latent image is developed by placing the imaging surfacein developing configuration with a patterned applicator surface having asubstantially uniform distribution of raised portions or lands anddepressed portions or valleys and containing a relatively non-conductiveliquid developer in the depressed portions thereof while the raisedportions are substantially free of developer.

CROSS REFERENCES TO RELATED APPLICATIONS This application is acontinuation application of US. Ser. No. 839,801, filed July 1, 1969,now abandoned.

BACKGROUND OF THE INVENTION This invention relates to imaging systems,and more particularly, to improved developer systems and techniques.

The formation and development of images on the surface ofphotoconductive materials by electrostatic means is well known. Thebasic xerographic process, as taught by C. F. Carlson in US. Pat.2,297,691 involves placing a uniform electrostatic charge on aphotoconductive insulating layer, exposing the layer to alight-and-shadow image to dissipate the charge on the areas of the layerexposed to the light and developing the resulting electrostatic latentimage by depositing on the image a finelydivided electroscopic materialreferred to in the art as toner. The toner will normally be attracted tothose areas of the layer which retain a charge, thereby forming a tonerimage corresponding to the electrostatic latent image. This powder imagemay then be transferred to a support surface such as paper. Thetransferred image may subsequently be permanently affixed to a supportsurface as by heat. Instead of latent image formation by uniformlycharging the photoconductive layer and then exposing the layer to alight-and-shadow image, one may form the latent image directly bycharging the layer in image configuration. The powder image may be fixedto the photoconductive layer if elimination of the powder image transferstep is desired. Other suitable fixing means such as solvent orovercoating treatment may be substituted for the foregoing heat fixingstep.

Similar methods are known for applying the electroscopic particles tothe electrostatic latent image to be developed. Included within thisgroup are the cascade development technique disclosed by E. N. Wise inUS. Pat. 2,618,552; the powder cloud technique disclosed by C. F.Carlson in US. Pat. 2,221,776 and the magnetic brush process disclosed,for example, in US. Pat. 2,874,- 063 Development of an electrostaticlatent image may also be achieved with liquid rather than dry developermaterials. In conventional liquid development, more commonly referred toas electrophoretic development, an insulating liquid vehicle havingfinely divided solid material dispersed therein contacts the imagingsurface in both charged 3,806,354 Patented Apr. 23, 1974 "ice anduncharged areas. Under the influence of the electric field associatedwith the charged image pattern the suspended particles migrate towardthe charged portions of the imaging surface separating out of theinsulating liquid. This electrophoretic migration of charged particlesresults in the deposition of the charged particles on the imagingsurface in image configuration. Electrophoretic development of anelectrostatic latent image may for example, be obtained by flowing thedeveloper over the image bearing surface, by immersing the imagingsurface in a pool of the developer or by presenting the liquid developeron a smooth surfaced roller and moving the roller against the imagingsurface.

A further technique for developing electrostatic latent images is theliquid development process disclosed by R. W. Gundlach in US. Patent3,084,043 hereinafter referred to as polar liquid development. In thismethod, an electrostatic latent image is developed or made visible bypresenting to the imaging surface a liquid developer on the surface of adeveloper dispensing member having a plurality of raised portions orlands defining a substantially regular patterned surface and a pluralityof portions depressed below the raised portions or valleys. Thedepressed portions of the developer dispensing member contain a layer ofconductive liquid developer which is maintained out of contact with theelectrostatographic imaging surface. Development is achieved by movingthe developer dispensing member loaded with liquid developer in thedepressed portions into developing configuration with the imagingsurface. The liquid developer is believed to be attracted from thedepressed portions of the applicator surface in the charged or imageareas only. The developer liquid may be pigmented or dyed. Thedevelopment system disclosed in US. Patent 3,084,043, differs fromelectrophoretic development systems where substantial contact betweenthe liquid developer and both the charged and uncharged areas of anelectrostatic latent imaging surface occurs. Unlike electrophoreticdevelopment systems, substantial contact between the polar liquid andthe areas of the electrostatic latent image bearing surface not to bedeveloped is prevented in the polar liquid development technique.Reduced contact between a liquid developer and the non-image areas ofthe surface to be developed is desirable because the formation ofbackground deposits is thereby inhibited. Another characteristic whichdistinguishes the polar liquid development technique fromelectrophoretic development is the fact that the liquid phase of a polardeveloper actually takes part in the development of a surface. Theliquid phase in electrophoretic developers functions only as a carriermedium for developer particles.

While capable of producing satisfactory images these liquid developmentsystems in general suffer deficiencies in certain areas and are in needof further development and improvement. Particularly troublesomedifiiculties are encountered in liquid development systems employing areusable or cycling electrostatographic imaging surface. In thesesystems an imaging surface such as a selenium or selenium alloy drumtype photoconductor is charged, exposed to a light and shadow image anddeveloped by bringing the image bearing surface into developingconfiguration with an applicator containing developing quantities ofliquid developer thereon. The liquid developer is transferred accordingto the appropriate technique from the developer applicator onto theimage bearing surface in image configuration. Thereafter, the developerpattern is transferred to a receiving surface such as paper. During thetransfer step not all the liquid developer is transferred and thereforea subsequent cleaning step is required.

In electrophoretic development the entire imaging surface is contactedwith the liquid developer with the charged particles separating from thecarrier liquid and migrating to the charged field or image portions. Theparticles strongly adhere to the imaging surface by means of van derWaals forces since the particles frequently come within about fivehundred angstroms of the imaging surface. The van der Waals forces areso strong that in the subsequent transfer step a considerable portion ofthe particles remain on the imaging surface thus producing prints ofrelatively low density. In addition to poor density with transfer, theadhering particles on the imaging surface drastically increase theeffort necessary to clean the residual developer from the imagingsurface and frequently require sufiicient cleaning to result indegradation of the photoconductor. In general, electrophoreticdevelopment in systems employing recycling electrostatographic imagingsurfaces provides low efficiency in both transfer of the developer to areceiving surface and in the cleaning step. Further more, since there isa general area contact by the liquid portion of the developer with theentire imaging surface some of the liquid vehicle will be present in thebackground areas of the final copy. When the liquid vehicle is avolatile vehicle some heat must generally be supplied to evaporate thelayer of vehicle in the background areas. On the other hand,non-volatile liquids generally produce prints with oily .or greasybackground areas. In either case, without a heating or fusing step thecopy paper is usually wet and the image area poorly fixed to the copypaper. Furthermore, the developers used in electrophoretic developmentare polarity sensitive. That is, they must be specially selected todevelop images of either positive or negative charge. In addition, inelectrophoretic development there may be a gradual depletion ofelectroscopic particles in the developer since they separate from thecarrier liquid during development. Any depletion may be significantsince generally only relatively small quantities of electroscopicparticles are present in the developer to ensure low viscosity of theliquid developer necessary for acceptable development speeds.

In the polar ink development systems disclosed in US. Pat. 3,084,043 thedeveloping liquid is relatively conductive having a resistivity lessthan ohm centimeters. After transfer of the developer in imageconfiguration from the electrostatographic imaging surface to areceiving surface and even relatively vigorous cleaning, a portion ofthis type of developer is also observed to remain on the imagingsurface. This developer residue is damaging to cyclical use of theimaging surface. Subsequent recharging of the photoconductor, forexample, may be inadequate since the conductive liquid may dissipate thecharge. Furthermore lateral conductivity of the liquid developer on thephotoconductor may become excessive and the resolution of the resultingimage will be poor.

In many of the developer materials employed in these techniqueshyperoscopic developer materials are employed which absorb atmosphericmoisture thereby lowering the developer viscosity and altering thedeveloping conditions. It is, therefore, clear that there is acontinuing need for a better liquid development system employing acycling or reusable photoconductor.

SUMMARY OF THE INVENTION It is, therefore, an object of this inventionto provide a developing system which overcomes the above noteddeficiencies.

It is another object of this invention to provide a liquid developmentsystem employing an electrostatographic imaging surface capable ofrepeated recycling.

It is another object of this invention to provide a liquid developmentsystem employing a cycling electrostatographic imaging surfacepermitting relative ease in cleaning.

It is another object of this invention to provide a liquid developmentsystem employing a reusable electrostatographic imaging surface whereinprints of improved resolution and/or density are obtained.

It is another object of this invention to provide a liquid developmentsystem which enables an electrostatographic imaging surface to be cycledrepeatedly.

It is another object of this invention to provide a liquid developmentsystem employing a photoconductor capable of repeated cycling withincomplete cleaning of the photoconductor on each cycle.

It is another object of this invention to provide an electrostatographicimaging system employing a non-volatile liquid developer which producesprints which are not wet, slippery or greasy in the background areas.

It is another object of this invention to provide an electrostatographicimaging system employing liquid developers which are not polaritysensitive and which are stable suspensions independent of particlesurface potential or concentration.

It is another object of this invention to provide liquid developershaving high pigment contents.

It is another object of this invention to provide a liquid developmentsystem superior to known systems.

The above objects and others are accomplished, generally speaking, byproviding an electrostatographic imaging system wherein an electrostaticlatent image is developed by placing the imaging surface in developingconfiguration with a patterned applicator surface having a substantiallyuniform distribution of raised portions or lands and depressed portionsor valleys and containing a relatively electrically non-conductiveliquid developer in the depressed portions thereof while the surfaces ofthe raised portions are substantially free of developer.

Development is obtained by placing the applicator surface sufficientlyclose to the electrostatographic imaging surface such that therelatively non-conductive liquid developer is pulled from the recessedportions of the applicator surface to the imaging surface in imageconfiguration. Generally to provide maximum image density it ispreferred to place the raised portions of the applicator surface inslight or gentle contact with the imaging surface provided that theraised portions are substantially free of liquid developer.

Any suitable applicator surface may be employed which has asubstantially uniform pattern of raised portions and depressed portionprovided that the depressed portions are sufficiently large to holddeveloping quantities of liquid developer therein. To minimize wear onthe imaging surface it is preferred to provide raised portions which areuniformly curved or substantially flat on the surfaces which contact theimaging surface.

Typical applicator surfaces include, among others, porous ceramics,metallic sponge, patterned webs or belts, capillary combs, andcylindrical rolls having surface patterns such as single screw cuts ortrihelicoid, pyramidal or quadragravure indentations. To provide goodimage resolution it is preferred that the applicator surface have apattern comprising between about and about 300' demarcations of raisedor depressed areas per inch. Generally, with more coarse patterns,insufficient resolution is obtained and with finer patterns insufficientloading of developer in the recessed portions is obtained to providegood image density. It is generally preferred to employ a pattern ofrecessed grooves such as in the trihelicoid pattern since this patternfacilitates better doctoring of the applicator surface.

The applicator surface may be loaded with developer in any suitablemanner. Typical developer loading techniques include applying developerfrom a roll or sponge roll or immersing the applicator in a bath. Priorto contacting the imaging surface, the applicator surface should bewiped or doctored clean to remove substantially all liquid developerfrom the raised portions of the applicator surface. Any suitable meansmay be provided as the doctoring device. Typical doctoring devicesinclude scraper blades and squeegee rolls. The doctoring in addition toremoving liquid developer from the raised portions of the applicatorsurface preferably provides a slight wiping action of the liquiddeveloper in the recessed portions of the applicator surface to therebymaintain the level of the liquid developer in the recessed portionsslightly below the level of the raised portions. Such a loading ofdeveloper on the applicator surface minimizes deposits in the non-imageareas.

Any suitable liquid developer composition may be employed which isrelatively electrically non-conductive. Typically, the developer has abulk resistivity greater than ohm centimeters and less than about 10 ohmcentimeters. In recycling systems, the more electrically conductive thedeveloper, the greater the opportunity for decreased charge retentionand increased lateral conductivity on the imaging surface and thereforepoor resolution. On the other hand, the more electrically resistive thedeveloper is, the greater the time constant for lateral discharge of theimage. Lateral discharge is of significance in a recycling system sincefor each imaging cycle there is generally a residue of developerremaining on the imaging surface from the preceding cycle and placing ofthe charged image on the imaging surface must take place with theresidual developer still on it.

The time constant T for the lateral discharge of the smallest imageelement 1'" to be resolved to its surroundings is determined by the wellknown relationship. 'r =pi/d, where p and d are respectively the bulkresistivity and thickness of the residual liquid developer film and fthe capacitance of area element 1' of the dielectric or photoconductor.For a resolution of 10 lp./mm., the area of element 1' is 10- cm For atypical 50 1. selenium photoconductor layer of dielectric constant 5:6,or a typical 25p. organic dielectric of e=3, i-10" farad/ element. Witha typical liquid developer film residue of the order of 1.0g. or 10- cm.in thickness, T -10 p sec. Therefore the time for 1-1/ (or 63%)discharge of an image element by lateral conduction through the liquidresidue film is given by Table 1:

Since at the present state of the art, practical electrostatic imagingrates require a latent image life of the order of at least about 1 sec.,and preferably 2 or 5 sec., it follows that liquid developers forpractical cycling photoreceptors should have a resistivity of at least10 ohm cm. as measured by conventional pulsed D.C. parallel plateconductivity cell method.

The rate of development is set by a similar time constant, but one whichinvolves the rate of charge induction through the liquid developer: fi ee p where 5 is the dielectric constant of the liquid, s the permitivityof free space (8.85 l0- fd./cm.) and p the resistivity of the liquid, asbefore. Values of q range generally from 2 to 6, with 3.4 a typicalexample, applicable to many mineral oil based liquid developers.Assuming an effective developing zone about 0.2 inch in width, theelectrical limitation or maximal development speed may be estimated as:u 0.20/ 1- (inches/ sec.)

TABLE 2 10 10 10 10 10 Ohmcm. 3.0 30 300 3,000 30,000 Millisee.

66 6.6 .66 .066 .0066 Inches/sec.

This theory indicates that high development rates may be achieved at pl0 ohm cm. However, actual experience indicates that it does not predictthe limiting velocities at p 10 ohm cm. very accurately. Instead, someadditional mechanism which is not yet clearly understood provides amaximal development speed greater than about 3 inches/sec. even at p=10-10 ohm cm. It follows that liquid developers are operable in the entireresistivity range cited (10 -10 ohm cm.). The preferred and mostpractical operating range of resistivity providing the balance betweenconductivity, time constant and development speed is from about 2x10 toabout 10 ohm cm.

An optimum balance between conductivity and time constant is generallyobtained with developers having a resistivity of from about 10 to about10 ohm centimeters.

The developers of this invention may include one or more liquidvehicles, colorants such as pigments and dyes, and dispersants. Inaddition, a variety of specialized agents may be employed for particularfunctions. For example, viscosity controlling additives or additiveswhich contribute to fixing a pigment on copy paper may be employed.

Any suitable vehicle contributing to the above properties may be used.Typical vehicles within this group that may be employed singly or incombination include mineral oil, vegetable oils, such as castor oil andits oxidized derivatives, peanut oil, coconut oil, sunflower seed oil,corn oil, rape seed oil, and sesame oil. Also included are mineralspirits, fluorocarbon oils such as du Ponts Freon solvents and Krytoxoils, silicone oils, kerosene, carbon tetrachloride, toluene, oleic acidand drying oils such as linseed oil and tung oil and highly purifiedpolypropylene glycol.

In addition to the above vehicles an auxiliary or secondary vehicle maybe employed to impart or adjust any one or more of the properties of theliquid developer. Any suitable material may be employed as the secondaryvehicle and may, for example, have dispersant properties, contribute toviscosity adjustment, or confer wetting properties to the pigmentemployed or act as a fixing agent. In addition, the secondary vehiclespreferably exhibit properties in common with the principal vehicle ofbeing nonodorous, non-hygroscopic, and of low volatility to provide astable developer with a non-offensive odor. An additional function ofsecondary vehicles may be to help the developer penetrate into the copypaper. Typical materials that may be employed as secondary or primaryvehicles include dibutyl phthalate, butyl isodecyl phthalate, butyloctyl phthalate, diisooctyl phthalate, di(2-ethyl hexyl) phthalate,isooctyl isodecyl phthalate, normal octyl decyl phthalate, diisodecylphthalate, ditridecyl phthalate, isodecyl tridecyl phthalate, diisooctyladipate, di(2-ethyl hexyl) adipate, isooctyl iosdecyl adipate, normaloctyl decyl adipate, diisodecyl adipate, diisooctyl sebacate, di Z-ethylhexyl sebacate, polyadipate ester, polyadipate ester, isooctylpalmitate, butyl stearate, butyl oleate, triethylene glycol dicaprylate,triethylene glycol caprylatecaprate, triethylene glycol dipelargonate,diethylene glycol dipelargonate, butanedial dicaprylate, triisooctyltrimellitate, tri 2-ethyl hexyl trimellitate and mixed normal trialkyltrimellitates.

Any suitable colorant may be employed in the developer including bothpigments and rlyes. It is preferred that the colorant be fast to lightin order to obtain image permanence. Typical pigments include carbonblack, charcoal and other forms of finely-divided carbon, quinacridones,phthalocyanine blues, iron oxide, ultramarine lblue, zinc oxide,titanium dioxide, and benzidine yellow. Typical dyes include IrgacetBlack 'RL, oil red, oil blue, oil yellow. Because of the relative easein being dispersed throughout the vehicle surface resinated pigmentssuch as those manufactured by CIBA under the tradename Microlith arepreferred. Microlith CT is a preferred resinated carbon black. Whiledyes may be employed it is generally preferred to employ pigmenteddevelopers since they provide 'better archival permanence, are morereadily immobilized at the point of deposition and provide higherdensity since they tend to be filtered out and remain closer to thepaper surface on the final print.

A dispersant is generally employed to aid in dispersing the pigment andother additives in the vehicles. Any suitable dispersant may be employedthat is compatible with and soluble in the vehicle, Typical materialsinclude alkylated polyvinyl pyrrolidone and wood rosin derivatives suchas Stabelite ester, manufactured by Hercules Powder Co. andinterpolymers of n-octodecyl vinyl ether and maleic anhydride such asGantrez AN 8194 manufactured by GAF Corp.

The proportions of the several constituents in the developer may bevaried over a wide range depending on individual properties of theconstituents and operational considerations of the specific developmentscheme. A significant factor in determining proportions is the speed ofdevelopment since with higher speeds lower viscosity developers must beused than at lower speed. One skilled in the art may readily determinethe appropriate viscosity for any given development speed.

in liquid development systems having development speeds of from about 5to about 20 inches per second, for example, developer viscosities offrom about 300 to about 1800 centipoises measured at 25 C. are preferredto provide ease of operation and desired print quality. Speeds of 200inches per second, on the other hand, require a lower viscosity,typically of the order of about 100 centipoises. The viscosity is inpart dependent on the pigment loading of the vehicle. At constantdispersant concentration as more pigment is added the viscosity of thedeveloper increases and operable development speed is lowered. Thebalance between pigment loading and dispersant concentration to obtainmaximum image density and development speed to maintain maximumdevelopment may be readily determined by one skilled in the art.

The several constituents may generally be present in a developer inamounts according to the following weight percentages:

Wt. percent Vehicle (including principal and secondary vehicle Fromabout 40 to about 90 Colorant: Pigment or dye Up to about 60 DispersantsUp to about 20 Typical developers have the following composition byweight:

Wt. percent Total vehicle From about 40 to about 85 Colorant: Pigment ordye From about 15 to about 60 Dispersants From about 1 to about 20Within the broad range of proportions set forth above a preferred rangeof proportions for the constituents of the developer providing goodprint quality and ease of operation are the following:

Wt. percent Total vehicle From about 65 to about 85 Primary vehicle Fromabout 20 to about 85 Secondary vehicle From about to about 45 ColorantFrom about 20 to about 50 Dispersant From about 5 to about 15Particularly preferred formulations in providing optimum image densityand resolution include a pigment loading of from about 30% to about 40%by weight.

The developers of this invention may be prepared by simply mixing theseveral constituents. However, to provide homogeneity it is generallypreferred to combine the constituents of the vehicle first while heatingthem and then adding dispersant, pigment or dye any any other additives.The pigment may be comminuted separately or together with the vehicle.In addition, suitable control, suspending and fixing agents as are wellknown in the art may be added in conventional manner.

Developmet of an electrostatic latent image according to the techniquedescribed herein may be obtained on any suitable electrostatographicimaging surface. Basically, any surface upon which an electrostaticcharge pattern may be formed or developed may be employed. Typicalelectrostatographic imaging surfaces include dielectrics such as plasticcoated papers, xeroprinting masters, and photoconductors. Typicalphotoconductors that may be employed include selenium and seleniumalloys, cadmium sulfide, cadmium sulfo selenide, phthalocyanine bindercoatings and polyvinyl carbazole sensitized with 2,4,7-trinitrofluorenone. The electrostatographic imaging surface may beemployed in any suitable structure including plates, belts or drums andmay be employed in the form of a binder layer coated on a substrate. Theimaging surfaces may be overcoated with suitable dielectric materials inconventional manner.

In the cycling electrostatographic imaging systems of the presentinvention it is generally necessary to cyclically clean the imagingsurface. Any suitable cleaning system may be employed. A typicalcleaning system scrubs the ink rfilm on the photoconductor surfaceobliterating the image pattern by smearing the developer over thesurface. The residual developer is subsequently picked up by anabsorbent web which may absorb the developer. For example, a squeegeeroller may be used as the scrubbing or obliterating device and anabsorbent web wrapped around a portion of the photoconductor drum andmoving slowly counter to the direction of rotation of the drum may beused.

The mechanism of development according to this invention is presentlybelieved to be substantially the same as that in the polar inkdevelopment technique described by R. W. Gundlach in US. Pat. 3,084,043.The liquid developer is applied to the patterned applicator such thatthe raised portions of the applicator surface are substantially free ofdeveloper and the level of liquid in the recessed portions of theapplicator is slightly below the level of the lands. Surface tensionretains the developer in cohesive configuration in the depressed portionof the applicator surface and as the raised portions of the applicatorsurface are placed in light or gentle contact with theelectrostatographic imaging surface the liquid developer in response tothe electrostatic field of force on the imaging surface creeps up thesides of the depressed portions of the applicator surface and depositson the imaging surface substantially only in accordance With the patternof electric charge. The developer remains in the depressed portions ofthe applicator surface except in those portions which are under theinfluence of the attracting electrostatic force.

The developer applicator is generally biased or directly connected toground through connection to a variable DC. potential source so that theliquid developer will be electrostatically attracted from the applicatorto the imaging surface in image configuration. When so biased, thecharges on the imaging surface induce equal and opposite charges in theliquid developer. For example, when the applicator is grounded and theimage surface carries a positive charge, negative charge is induced inthe liquid developer adjacent the positive charges and the developermoves toward the imaging surface in response to the electrostatic fieldgenerated between these charges. Portions of the imaging surfacecarrying no charge, induce no charge in the developer and thus thedeveloper is not pulled out of the recessed portions of the applicatorsurface to non-field areas of the image surface.

Reversal development may be obtained by applying to the developerapplicator a potential of the same polarity and of the same amount asthe charged areas on the imging surface to cancel out the field atcharged areas and provide an electrostatic field between the unchargedareas of the imaging surface and the developer on the applicatorsurface. Again a charge is induced in the de- 'veloper in response tothe electrostatic field and the developer creeps up the recessedportions of the applicator surface adjacent areas of the imaging surfacewhich are uncharged.

This is substantially the mechanism of development despite the fact thatthe developer is relatively non-conductive. The developers employed inthis development mechanism are not polarity sensitive. That is, unlikeclassical electrophoretic developers, they are equally effective indeveloping positively, as well as negatively charged patterns on theimaging surface, the difference being only in the polarity of chargeinduced in the developer. Further, unlike electrophoretic development,migration of charged particles from the insulating carrier liquid playsno significant role. Instead, charge is induced in the entire developerwhich migrates substantially intact from the depressed portions of theapplicator surface to the imaging surface. While particle migration maynot be totally inhibited, if present, it is present in an insignificantinsubstantial amount. This mechanism is generally substantiated by thefact that in development according to the disclosed technique the liquiddeveloper is readily transferred and cleaned from the imaging surfaceand there is no evidence of the deposition of pigment particles out ofthe developer on the imaging surface. The additional observation thatthe developed image obtained according to the instant techniquecomprises both pigment particles and carrier liquid in substantially thesame relative proportions as present in the original developer sup plywhereas a developed image obtained through electrophoretic developmentcomprises substantially only the solid particles which have separatedfrom the carrier liquid is further evidence of the differences betweenconventional liquid development and the described improved developmenttechnique.

DESCRIPTION OF PREFERRED EMBODIMENTS The liquid developer employed inthis example is of the following composition by weight and has a bulkresistivity of about 1.5 (ohm cm.) and a dielectric constant of about3.20.

Parts Drakeol 9 30 Ganex V216 Microlith CT 18 Methyl violet tannate 3Paraflint RG wax 0.5

Drakeol 9 is a light mineral oil manufactured by Pennsylvania RefiningCo. Ganex V216 is an alkylated polyvinyl pyrrolidone compoundmanufactured by GAF which serves as a pigment dispersant and may also beregarded as a vehicle. Microlith CT is a resinated pre-dispersed carbonblack pigment composed of about 40% carbon black pigment and 60% estergum resin manufactured by CIBA. Paraflint R6 is a hard synthetic waxavailable in flake form from Moore and Munger.

The developer is prepared by combining the mineral oil and Ganex V216 ina suitable vessel while stirring, heating to about 100 C. and thenadding the pigment and other ingredients while continuing the stirring.

The developer is applied to a cylindrical applicator roller having atrihelicoid pattern of about 150 lines per inch cut at an angle of about45 to the longitudinal axis and to a depth of about 2.5 mils. The ridgesof the applicator surface are substantially fiat and the roller is wipedwith a polyurethane doctor blade having a Shore A hardness of durometersto remove substantially all developer from the ridges and provide alevel of developer in the grooves slightly below the level of theridges.

An electrostatic latent image is formed on a clean selenium xerographicplate comprising a surface layer of selenium about 50 microns thick on aconductive aluminum plate in conventional manner and the image isdeveloped by moving the applicator roller over the selenium plate at aspeed of about 10 inches per second, such that the edges are just incontact with the surface of the plate. A clearly defined developed imageis observed on the selenium plate. The developer is transferred to bondpaper in image configuration. The resolution of the print obtained isabout 10 line pairs per millimeter and image density is 1.0 withbackground less than 0.01.

Example 11 The procedure of Example I is repeated except that after thefirst print is obtained the selenium plate is manually cleaned with acotton cloth to remove substantially all the residual developer.Thereafter, the selenium plate is charged, exposed, developed and thedeveloped image transferred to paper in the same manner as in Example I.This sequence is repeated for 15 cycles. The resolution of the printsobtained is observed to gradually decrease from about 10 line pairs permillimeter on the first print to 8 l.p.lmm. for the second, 4 1.p./mm.for the fourth, 3 l.p.lmm. for the tenth and 1 l.p./mm. for thefifteenth. Image density remains nearly constant at about 1.0 withbackground at about 0.01 for all prints.

Example III The procedure of Example I is repeated except that thedeveloper is applied by totally immersing the selenium plate in a bathof developer. No image is developed on the plate. The plate is uniformlycoated with developer.

Example IV The procedure of Example I is repeated with a developerhaving the following composition by weight and having an electricalconductivity of about 1.4 10 (ohm cm.)" and a dielectric constant ofabout 39.5.

Santicizer 160 is butylbenzyl phthalate manufactured by Monsanto.

The transferred print obtained on bond paper has good image density andresolution of about 5 line pairs per millimeter. The ink residueremaining on the selenium photoreceptor is substantially removed byrubbing the plate surface with absorbent cotton but still leaving a thinlayer of ink film on the plate surface. The plate is now re-charged,exposed and passed through the developer as in Example I. No image isdeveloped on the plate. The plate is now thoroughly cleaned of all inkfilm residue using soap and water and drying the plate by forced airdrying for about one hour at F. The plate is again charged, exposed anddeveloped. The image now obtained on bond paper has good density and aresolution of about 3 line pairs per millimeter.

Example V A paper backed zinc oxide binder layer photoconductor ischarged and exposed in conventional manner. The electrostatic latentimage is developed with a developer having the following composition byweight.

Parts Drakeol 9 38 Microlith CT 38 Rucoflex TG-8 9 Ganex V216 14 Drakeol9 is a mineral oil manufactured by Pennsylvania Refining having akinematic viscosity of about 15.7-18.1 centisto-kes at 25 C. and aspecific gravity of 0.84. Rucoflex TG-8 is triethylene glycoldicaprylate manufactured by Hooker Chemical Company which serves as asolvent for the resinated carbon black pigment and may be regarded as asecondary vehicle in this formulation. The liquid developer has anelectrical resistivity of about O.7 10 ohm cm. and is applied to thepatterned applicator roll in the manner described in Example I.Development is obtained in the manner of Example I. The resolution ofthe developed image is observed to be about 8 line pairs per millimeterwith good image density. The background areas of the zinc oxide paperare dry and not oily.

Example VI The procedure of Example V is repeated except that the zincoxide paper bearing the electrostatic latent image is immersed in theliquid developer. No image is developed on the paper. The zinc oxidepaper is uniformly coated with developer.

Example VII The procedure of Example I is repeated with a developerhaving the following composition by weight and having an electricalconductivity of about 3 10- (ohm cm.)" and a .dielectric constant ofabout 2.2.

Parts Oleic acid (USP grade) 75 Light resistant molybdate orange deep 25Light resistant molybdate orange deep is a flushed predispersed pigmentcomposed of about 75% pigment, 22% of a medium oil soya alkyd varnishand 3% mineral spirits manufactured by the Sherwin-Williams Co.

The transferred print obtained on bond paper has good image density andresolution of about line pairs per millimeter. The selenium plate iscleaned as described in Example II and thereafter charged, exposed anddeveloped for several cycles. After cycles the resolution of the printobtained is 3 line pairs per millimeter.

Example VIII A xeroprinting master is prepared by placing a thininsulating coating of epoxy resin about 0.0005 inch thick in imageconfiguration on conductive plate of aluminum. The plate is charged to+450 volts by passing it under a corona charging unit. The image isdeveloped with the liquid developer and in the manner described inExample I. The developer is transferred to bond paper and the resultingprint has image density of about 1.1, background density of 0.01, and aresolution of about 5 line pairs per millimeter. The plate is cleaned asdescribed in Example II. Thereafter, the xeroprinting master isrepeatedly charged, developed and the developer transferred to bondpaper as described above for 25 cycles. The speed of development isabout 12 inches per second and the 25th print obtained is ofsubstantially the same quality as the first print.

Example IX Example VIII is repeated except that the xeroprinting masteris replaced by a photoconductor comprising a 20 micron layer of seleniumon an aluminum substrate overcoated with a quarter mil film ofpolyethylene terephthalate (obtained from E. I. du Pont de Nemours & Co.under the trade name Mylar) made according to the technique described inExample I of US. Pat. 3,251,686. Results similar to those obtained inExample VIII are observed on repeated cycling.

Examples 1, II and VIII demonstrate the cycling ability obtained whenemploying the liquid developers and techniques of the invention. Printresolution of 5 line pairs per millimeter is generally recognized asvery acceptable quality. Examples 1H and VI demonstrate that developmentaccording to a conventional electrophoretic development technique is notpossible with the specified compositions. Example IV demonstrates thatwith a fairly conductive liquid developer recycling is impossible. Thepresence of the conductive developer residue on the imaging surfacepermits lateral discharge during charging and exposure to such an extentthat there is a complete loss of resolution. Examples V and VIdemonstrate that development with materials of very high pigment loadingis possible according to the technique of this invention and notpossible according to a conventional electrophoretic technique. In thexeroprinting mode of Example VIII since the image is the same for eachcycle the cleaning step may be dispensed with.

As may also be observed from the foregoing description, examples andembodiments the materials and techniques of the instant invention haveseveral advantages over known development systems. Development with aliquid developer may be obtained in a system employing a reusablephotoconductor or other reusable electrostatographic imaging surface.The instant invention makes available as developers an additional groupof nonvolatile materials and permits rapid development without a dryingor heat fixing step. Unlike conventional electrophoretic development thebackground areas on the copy prints are not in contact with thedeveloper and therefore remain dry and nongreasy and do not require afusing step to fix the developer to the copy print. Further, since thereis substantially no particle migration the composition of the developerin a developer supply does not have to be frequently monitored andadjusted and the developer image is not as closely held to the imagingsurface, and, therefore, more readily transferred to a copy print andmore readily cleaned.

Although specific materials and operational techniques are set forth inthe above exemplary embodiments using the developer composition anddevelopment techniques of this invention these are merely intended asillustrations of the present invention. There are other developermaterials and techniques such as those listed above which may besubstituted for those in examples with similar results.

Other modifications of the present invention will occur to those skilledin the art upon a reading of the present disclosure which modificationsare intended to be included within the scope of this invention.

What is claimed is:

1. The method of cyclically developing electrostatic latent imagepresent on a reusable electrostatographic imaging surface comprising thesteps of forming an electrostatic latent image on said reusable imagingsurface, providing an applicator having a substantially uniform patternof raised portions and depressed portions, said depressed portionscontaining developing quantities of an electrically non-conductiveliquid developer having a bulk resistivity of from about 10 ohm-cm.'toabout 10 ohm-cm. while said raised portions are substantially free ofliquid developer, positioning said applicator adjacent said imagingsurface so as to induce equal and opposite charges in the liquiddeveloper in regions corresponding to those areas of the imaging surfaceintended to be developed, such that the liquid developer iselectrostatically pulled from the applicator to the imaging surface inimage configuration, transferring said liquid developer from saidimaging surface to a receiving surface, preparing said reusable imagingsurface for the next imaging sequence and repeating the steps offorming, providing, positioning and transferring at least one additionaltime whereby residues of said non-conductive liq- 13 uid developerremaining on the imaging surface are not damaging to cyclical use of theimaging surface.

2. The method of claim 1 wherein said applicator is electrically biased.

3. The method of claim 1 wherein the electrostatographic imaging surfaceis a substrate backed photoconductive insulating binder layer.

4. The method of claim 1 wherein the electrostatographic imaging surfaceis a paper backed dielectric layer.

5. The method of claim 1 wherein the electrostatographic imaging surfacecomprises a photoconductor seselected from selenium or selenium alloys.

6. The method of claim 1 wherein the electrostatographic imaging surfaceis a xeroprinting master.

7. The method of claim 1 wherein the electrostatographic imaging surfaceis a phthalocyanine binder layer.

8. The method of claim 1 wherein the liquid developer has an electricalresistivity of from about 2x10 ohm centimeters to about 10 ohmcentimeters.

9. The method of claim 1 wherein the liquid developer has an electricalresistivity of from about 10 ohm centimeters to about 10 ohmcentimeters.

10. The method of claim 1 wherein said liquid developer is non-volatile.

11. The method of claim 1 wherein the developer during said positioningis substantially free of particle separation.

12. The method of claim 1 wherein pigment is present in the developer inan amount of from about 15 percent to about 60 percent by weight of theentire developer composition.

13. The method of claim 1 wherein pigment is present in the developer inan amount of from about percent to about 50 percent by weight of theentire developer composition.

14. The method of claim 1 wherein pigment is present in the developer inan amount of from about percent to about percent by weight of the entiredeveloper composition.

15. The method of claim 1 wherein the non-conductive liquid developerremains substantially homogeneous in composition throughout the severalstages of development until it is transferred to the final receiversheet.

16. The method of claim 1 wherein the applicator surface has a patternof from about to about 300 demarcations per inch.

17. The method of claim 1 wherein the applicator surface comprises apattern cylindrical roll.

18. The method of claim 17 wherein the pattern is a trihelicoid pattern.

19. The method of claim 17 wherein the pattern is a single spiral groovepattern.

20. The method of claim 1 wherein a potential of the same amount andpolarity as that of the charged areas of the imaging surface is appliedto the applicator.

21. The method of claim 1 wherein the electrostato graphic imagingsurface is a dielectric overcoated photoconductor.

22. The method of claim 1 wherein the electrostatographic imagingsurface is zinc oxide.

References Cited UNITED STATES PATENTS 3,469,978 9/1969 Wood et al. 96l.5 3,508,961 4/1970 Mahino et al. 961.5 3,669,073 6/1972 Savit et al.1l7-37 LE 3,084,043 4/ 1963 Gundlach 117-37 LE 3,383,209 5/1968 Cassierset a1 117--37 LE 3,425,829 2/1969 Cassiers et al. 117-37 LE 2,878,1203/1959 Mazer et al. 117-37 LE 3,196,013 7/1965 Walkup 117-37 LE3,276,424 10/ 1966 Marx et al. 1l737 LE 3,276,896 10/ 1966 Fisher 117-37LE 3,355,288 11/1967 Matkan 117-37 LE 3,391,014 7/1968 Fauser 11737 LE3,522,181 7/1970 Garrett et a1 117-37 LE FOREIGN PATENTS 790,013 7/ 1968Canada 11737 LE WILLIAM D. MARTIN, Primary Examiner M. SOFOCLEOUS,Assistant Examiner US. Cl. X.R. 96-4 LY, 1.4

